The hardened building materials discharged by the inventive apparatus serve, in general terms, to line or form parts of a structure. The invention is therefore applicable not only to sprayed or air-applied concrete or mortar but also to other building materials with different compositions. These include, for instance, the sprayable plasters. Pneumatically applied hydraulic building materials are also used below ground, i.e., in mining and tunnel construction. The substances used here often range from grainy to powdery and have different water-to-solids ratios. When being sprayed these substances are often handled with aggregates consisting of synthetic material or fiber mixtures, for example.
The invention is applicable, for instance, to air-applied concretes or mortars made of these building materials, which are in turn applied several centimeters thick, to the stone linings of underground tunnels. The invention is especially useful in applying such building materials to sloping vertical and overhead ceiling surfaces of underground tunnels, as soon as these surfaces are formed by blasting, or the like, in order to increase the inherent bearing strength of the surrounding ground. The inventive apparatus is also used for sealing fire dams and ventilation dams and applying the finish surface on walls to increase their weathering resistance, and in general for lining application. For the purpose of obtaining early strength, liquid activator is mixed into the building material to activate it to harden and thereby ensure optimum bearing strength in as short a time as possible which, in underground working, for instance, keeps the contraction of the strata low. The activator is often water glass, which accelerates the hardening process at varying rates depending on the dose.
Compared with the likewise known methods of dry conveyance, in which the necessary mixing water and activator are added to the dry building material at the discharge end of the gunning apparatus, the hydromechanical conveyance of the wet building material, in particular in the form of mortar or concrete, involves the advantage that the applied layers have an even composition in accordance with a given recipe, which avoids the fluctuations in strength in the applied layers resulting from uneven compositions of the building material and controlled addition of water.
In the conventional hydromechanically conveyed, pneumatically discharged application process, the building material is combined with water in a batch process remote from the spraying apparatus and conveyed to the apparatus through a conveyor line under the power of a positive displacement pump or the like. The building material is discharged with the aid of a discharge pipe, preferably provided with a nozzle, connected to the conveyor pipe through which the liquified mixture is conveyed. The surface to be coated is sprayed, being provided with a coat or a backfill by directing the flexible conveyor line portion. This directing is generally performed by an operator by hand.
In one variation of this process, the building material is put in suspension and accelerated with the aid of the compressed air added via nozzles. If an activator or hardener is to be added, it must be done in such a way that the building material begins to harden only when the building material has been discharged and has reached the surface of the structure. It is therefore favorable to add the activator to the building material at the discharge end of the gunning apparatus, if possible. However, this often means that the activator does not mix completely and homogeneously with the building material. As a result, the layer created on the surface in question by direction of a flexible portion for the conveyer line, turns out to be non-homogeneous and the early strength is thus not obtained uniformly. An additional problem of adding the activator at the discharge end of the gunning apparatus is the possibility that the activator liquid may not completely combine with the building material and be discharged to the atmosphere with the delivery air which leads to undesirable pollutant concentrations in the atmosphere.
This incomplete combination of the activator and the building material may also lead to rebounding losses, this term referring to the percentage of discharged building material which does not stick to the surface of the structure and falls off. It is true that the ranges of 30 to 40% ascertained for dry methods in underground working are not experienced by the wet methods to which the invention relates. However, there are many factors that influence rebounding losses. It depends, for instance, on the adhesive power of the building material, the angle of impact of the stream of building material discharged from the discharge opening, and similar parameters. In particular, systematic changes in the bearing strength of the base hit by the building material being sprayed constitutes one of the essential causes for rebounding. Regardless of the hardness of the impact, for example on a rock surface, the resistance of the base changes as the sprayed layer builds up, and is generally smaller the more the coat grows. The early strength of the building material therefore plays a part in this connection, as does the amount of building material discharged in a particular case. Rebounding losses can be limited to a large extent, in particular on overhangs, by providing a complete and homogeneous mixture of a hardener or activator with the building material discharged by the apparatus.